1. Field of the Invention
The present invention relates generally to a drain mechanism for a hydraulic power transmission joint for use in 4 wheel-drive mechanisms for motor vehicles, and more particularly to a drain mechanism for a hydraulic power transmission joint for switching from 4 wheel-drive to 2 wheel-drive depending on a rise in temperature.
2. Description of the Related Arts
Such a hydraulic power transmission joint for use in an automobile 4 wheel-drive mechanism is hitherto known from U.S. Pat. Nos. 5,706,658 and 5,983,635.
FIGS. 1 to 3 illustrate an exemplary drain mechanism for a hydraulic power transmission joint being currently developed by the present inventors on the basis of such United States Patents. This drain mechanism serves to perform a drainage from a high-pressure chamber to a low-pressure chamber when the temperature of oil has reached a predetermined level, to substantially nullify the transmission torque for the switching to 2 wheel-drive.
Referring to FIG. 1, a valve block 101 is coupled to a rotor (not shown), for integral rotation with the rotor. The rotor is spline coupled to a main shaft directly connecting with rear wheels, for integral rotation with the main shaft. The valve block 101 is formed with a high-pressure chamber 102 that communicates with a discharge hole formed in the rotor. The valve block 101 is formed with an orifice (not shown) that leads to the high-pressure chamber 102. A thermo-switch 104 is screwed by a securing member to the end face of the valve block 101 in such a manner that the thermo-switch 104 is inserted into the high-pressure chamber 102. The valve block 101 is provided with a drain hole 105 that opens to the high-pressure chamber 102. The valve block 101 is further provided with an accommodation hole 106 that communicates via the drain hole 105 to the high-pressure chamber 102. A check ball 108 for blocking the drain hole 105 is seated on a valve seat 107 formed in the accommodation hole 106. The check ball 108 is urged by a return spring 109. The valve block 101 is formed with a drain passage 110 that leads to the accommodation hole 106. The thermo-switch 104 has a head pin 111 adapted to be inserted into the drain hole 105 so as to press the check ball to open the drain hole 105. The thermo-switch 104, when a predetermined temperature is reached, causes the head pin 111 to extend to press the check ball 108 to open the drain hole 105, as shown in FIG. 2, allowing oil to be drained through the drain passage 110 into the low-pressure chamber to thereby substantially nullify the transmission torque for the switching to 2 wheel-drive. On the contrary, when the temperature at the thermo-switch 104 falls below the predetermined level, the check ball 108 is pushed back by the biasing force of the return spring 109, allowing the head pin 111 to return to its original position, to block the drain hole 105. The drainage of oil is thus ceased, allowing a return to 4 wheel-drive based on the torque transmission.
FIG. 3 depicts another drain mechanism. A valve body 112 is integrally formed with the fore-end of the head pin 111 of the thermo-switch fixedly secured to the valve block 101. The valve body is urged by the return spring 109 to block the drain hole 105. When a predetermined temperature is reached, the head pin 111 is extended rightward so that the valve body 112 is displaced rightward against the return spring 109, to thereby open the drain hole 105. Oil flows from the high-pressure chamber 102 through the drain hole 105 into the accommodation hole 106 that accommodates the return spring 109, after which it is drained through the drain passage 110 into the low-pressure chamber. When the temperature falls below the predetermined level, the return spring 109 biases the valve body 112 to block the drain hole, thus ceasing the drainage of oil.
However, such a drain mechanism for the hydraulic power transmission joint may often suffer from a problem that the thermo-switch tends to have a low strength-related durability since it is housed in the high-pressure chamber. Furthermore, due to the structure in which the check ball is thrust by the head pin of the thermo-switch, the diameter of the drain hole needs to be increased in order to allow for the passage of the head pin therethrough, thus subjecting the check ball to a larger load. If the check ball has a large pressure receiving area, it may be possible to deal with the high pressure by adding to the biasing force of the return spring, although another problem still remains that it may be difficult to raise the pressure to a higher level since the biasing force acquired by the return spring is limitative. Moreover, there was a need for elements for fixing the thermo-switch to the valve block, resulting in an enlarged scale of the drain mechanism.
The present invention provides a drain mechanism for a hydraulic power transmission joint ensuring an improved strength and durability of a thermo-switch and achieving a high-pressure setting by a return spring, as well as a reduction in size.
According to an aspect of the present invention there is provided a drain mechanism for a hydraulic power transmission joint adapted to be interposed between an input shaft and an output shaft that are rotatable relative to each other, to transmit torque as a function of the rotational-speed difference between the input and output shafts, the drain mechanism comprising a drain plug accommodated within a valve block and having a drain hole through which oil is introduced from a high-pressure chamber; a thermo-switch located within a low-pressure chamber in the valve block in such a manner as to be urged by a return spring, the thermo-switch having a head pin that protrudes when a predetermined temperature is reached; a fixed pin fixedly inserted into the valve block, the fixed pin being positioned so as to allow a head pin of the thermo-switch urged by the return spring to abut thereagainst, the fixed pin when the head pin protrudes at the predetermined temperature causing the thermo-switch to retreat against the return spring; and a drain pin slidably located within the drain plug, the drain pin being pressed by the thermo-switch to close the drain hole for the duration in which the predetermined temperature is not reached, the drain pin when the predetermined temperature is reached, opening the drain hole as a result of cutoff of the pressing force caused by a retreat of the thermo-switch. In this manner, the present invention allows the thermo-switch to be displaceably arranged in the low-pressure chamber, so that no high pressure acts on the thermo-switch, which contributes to an improvement in the strength and durability. Furthermore, there is no need to insert the head pin of the thermo-switch into the drain hole, with the result that the diameter of the drain hole can be reduced, enabling the hydraulic pressure at the high pressure side to be set at a higher level. In addition, there is no need for specific members for fixing the thermo-switch, making it possible to achieve a miniaturization while keeping the same performance.
Herein, the drain pin may have a recessed portion which is formed at its rear end side and which is open rearward, the recessed portion receiving the fixed pin that extends therethrough, with the head pin of the thermo-switch being inserted into the recessed portion so as to allow the head pin to abut against the fixed pin. This contributes to a secure restriction of the displacement of the drain pin, as well as to a secure operation of the thermo-switch.
The hydraulic power transmission joint employing the drain mechanism of the present invention comprises a housing coupled to the input shaft and having a cam face formed on its inner side surface; a rotor coupled to the output shaft and being rotatably accommodated in the housing, the rotor having a plurality of axially extending plunger chambers; a plurality of plungers each being reciprocatively accommodated in each of the plurality of plunger chambers under a biasing force of the return spring, the plurality of plungers being operated by the cam face upon the relative rotations of the input and output shafts; and a discharge hole formed in the rotor and opening to the plurality of plunger chambers; wherein the valve block is coupled to the rotor so that oil discharged by the operation of the plungers induces a reaction force in the plungers due to a flow resistance upon the passage through an orifice provided in the valve block, to thereby transmit torque between the housing and the rotor.
The above and other objects, aspects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.